This invention relates to normally tacky and pressure-sensitive adhesive compositions and to pressure-sensitive adhesive tapes made therewith. The invention is particularly concerned with pressure-sensitive adhesive products capable of adhering firmly to plasticized vinyl substrates, both initially and after contact over an extended period of time.
Polyvinyl chloride (or, as it is more commonly referred to, "vinyl") is one of the most versatile and widely used synthetic polymers available today. In its unmodified form, it is rigid and capable of being fabricated into containers, structural components, tubing, etc. When compounded with a plasticizer, it can be calendered into flexible sheet material that can be used as upholstery fabric, floor covering, pressure-sensitive adhesive tape backing, rainwear, etc., or extruded as electrical insulation, auto body side moulding, etc. Plasticized vinyls typically contain 15-50% by weight of either a mononeric or a polymeric plasticizer; because the monomeric plasticizers are less expensive, they are more commonly employed. Unfortunately, however, the monomeric plasticizers are of relatively low molecular weight, tending not only to migrate to the surface of the plasticized vinyls but also to volatilize therefrom, gradually embrittling the originally flexible product.
The tendency of a plasticizer to migrate into materials with which it comes in contact is also well known; see, e.g., U.S. Pat. No. 4,284,681. Thus, when a pressure-sensitive adhesive is applied to a plasticized vinyl substrate and allowed to remain in contact with it for an extended period of time, plasticizer from the vinyl tends to migrate into the pressure-sensitive adhesive, softening it and causing a decrease in adhesion; see, e.g., U.K. Pat. No. 1,051,125. Attempts have been made to solve the plasticizer migration problem by interposing an impenetrable barrier between the vinyl and the pressure-sensitive adhesive; see, e.g., U.S. Pat. Nos. 4,045,600 and 4,605,592. Others have developed pressure-sensitive adhesives that are said to be less susceptible to weakening by plasticizer; see, e.g., Graziano et al, Development of Acrylic Latex Pressure Sensitive Adhesive for Plasticized PVC films, Journal of Plastic Film & Sheeting, Vol. 2, April 1986, pp. 95-110. A commercially available tape that is widely used for adhering to plasticized vinyl utilizes a pressure-sensitive adhesive that is a terpolymer of 2-ethyl hexyl acrylate, vinyl acetate, and ethyl acrylate.
Another attempt to prevent migration of plasticizer from a plasticized vinyl substrate into a confronting pressure-sensitive adhesive has been to incorporate plasticizer into the pressure-sensitive adhesive, thereby minimizing the plasticizer gradient between the contacting layers; see, e.g., U.S. Pat. No. 4,272,573, U.K. Patent Application No. 2,162,191-A, and European Patent Application No. 150,978.
U.S. Pat. No. 4,753,846, issued June 28, 1988, discloses adhesive compositions suitable for application to plasticized vinyl comprising 30-70% of a vinyl ester or an alkenoic acid, 10-30% by weight ethylene, 20-40% by weight of a di-2-ethylhexyl maleate or di-n-octyl maleate or fumarate, and 1-10% of a mono-carboxylic acid.
Copolymers of acrylonitrile and butadiene are described in a B. F. Goodrich booklet entitled "Materials for Adhesives" dated March 1981, which states that when nitrile rubber is used in an adhesive, it should be combined with tackifiers, softeners, resins or other rubbers (pages 5, 8, 9 and 47) and often is combined with curing agents such as an isocyanate prepolymer (page 11).
In U.S. Pat. No. 4,212,912, an adhesive layer comprising an acrylonitrile/butadiene copolymer employs an organic peroxide to crosslink the polymer.
Despite the limited success achieved by products of the type discussed in preceding paragraphs, there has remained a strong commercial desire for a pressure-sensitive adhesive that was more effective, particularly one that not only initially adhered strongly to plasticized vinyl but which also retained a high degree of adhesion after remaining in contact with it for extended periods of time. In particular, none of the prior art products have been able to provide excellent resistance to shear after prolonged contact with plasticized vinyl.
Applicant has now discovered that certain acrylic copolymers comprising an alkyl acrylate monomer and a polar nitrogen-containing vinyl monomer photopolymerized in mixture with a nitrile butadiene rubber has surprising resistance to shear on plasticized substrates, including plasticized vinyl.